The core of a conventional doorbell is an electromagnet. If you've read How Electromagnets Work, you know that an electromagnet is a coil of wire, often wrapped around a piece of magnetic metal.
Electromagnets work on a very simple principle. Running electrical current through wire creates a very small magnetic field around the wire. Coiling the wire amplifies this magnetic field, so it has a substantial effect on any magnetic objects around it.
Just like the magnetic field around a permanent magnet, the magnetic field of an electromagnet has a polar orientation -- a "north" end and and a "south" end -- and it is attracted to iron objects.
When you hold down a doorbell button, it closes an electrical circuit so that household current flows through the electromagnet (or electromagnets) by way of a transformer. The transformer is a simple device that takes the 120-volt household current and steps it down to a 10-volt current (see Inside a Power-Cube Transformer to find out how). This current is then passed through the electromagnet wire.
The magnetic field of the electromagnet is put to work to drive some sort of a noisemaking apparatus. There are a number of different ways to configure the doorbell components in order to produce different sorts of noises. In the next couple of sections, we'll look at three common systems: the buzzer, the bell and the chime.